A Redox Auxiliary Strategy for Pyrrolidine Synthesis via Photocatalytic [3+2] Cycloaddition

Abstract Cycloaddition reactions can be used to efficiently assemble pyrrolidine rings that are significant in a variety of chemical and biological applications. We have developed a method for the formal cycloaddition of cyclopropyl ketones with hydrazones that utilizes photoredox catalysis to enable the synthesis of a range of structurally diverse pyrrolidine rings. The key insight enabling the scope of photoredox [3+2] cycloadditions to be expanded to C=N electrophiles was the use of a redox auxiliary strategy that allowed for photoreductive activation of the cyclopropyl ketone without the need for an exogenous tertiary amine co‐reductant. These conditions prevent the deleterious reductive ring‐opening of the cyclopropyl substrates, enabling a range of less‐reactive coupling partners to participate in this cycloaddition.